Liquid-crystalline polymers which can be processed from the melt are already known and described in "Polymer Liquid Crystals", Ciferi, Krigbaum, Meyer, Academic Press, 1982, and "High Modulus Polymers", Zachariades, Porter, Marcel Dekker Inc., 1988. Polymers of the type mentioned are usually fully aromatic polyesters, such as polyester carbonates, polyester amides, polyester imides and similar polymers or block polymers having a block based on these systems. A known method of measurement by means of which the existence of a liquid-crystalline melt can be detected is polarization microscopy, which is also described in the relevant literature.
Liquid-crystalline melts have the property that the molecular orientation of the polymer main chain in them can be achieved in a particularly simple manner by various methods, for example mechanically or by applying electric or magnetic fields, which makes it possible to obtain moldings having strongly increased mechanical properties in one or in more preferred orientations. This behavior of liquid-crystalline polymers is important for their further processing by the process of injection-molding but it also makes them in particular suitable for the preparation of fibers and films. If the melt of a liquid-crystalline polymer is extruded, for example from the sheet die of a conventional film extrusion unit, an orientation of the polymer in machine direction (MD), which can be further reinforced by a higher take-off rate for the melt from the die, can already be observed.
U.S. Pat. No. 4,161,470 describes the extrusion of liquid-crystalline polymers, such as, for example, of polycondensation products prepared from hydroxybenzoic acid and hydroxynaphthoic acid, to give fibers and films. A particular problem with the extrusion of liquid-crystalline polymers is that very often a kind of flow irregularity in the form of particles is observed, which could be designated as point-like morphological changes and which in any case cannot be removed by a conventional melt filtration. Upon orientation of the liquid-crystalline polymer, these flow irregularities are repeatedly the cause of thin places and in extreme cases even holes in the case of films or breakings in the case of fibers. These flow irregularities are thus responsible for the presence of weak or "predetermined" breaking points, which further intensifies one disadvantage of liquid-crystalline polymers, their tear propagation strength, which is usually low anyway. Apart from the mechanical properties, electrical properties and barrier properties are substantially affected by the flow irregularities.